When a railway vehicle runs on a curved track, a steering device of a truck of the railway vehicle turns two axles, arranged in the front and rear of the truck, in a yawing direction. The object of this turning is to reduce a turning resistance (lateral pressure) acting on the wheels attached to the axles.
The steering devices currently in commercial use turn the two axles symmetrically in the front and rear. Moreover, these steering devices set a steering angle of the axles to an angle that is geometrically most ideal (hereinafter, “radial steering angle”).
Referring to FIG. 14, assuming a steering angle to be “β”, a radius of curvature of the curved track to be “R”, and a distance between a center of a truck 2 and a center of axle 3 to be “a”, the radial steering angle, which is a steering angle at which the wheels attached to the axles will be in the most ideal steering state when running on the curved track, can be represented by the following Equation 1. In FIG. 14, 1 represents a vehicle body and 4 represents a track.β=sin−1(a/R)  [Equation 1]
However, when the truck is running on the curved track, the actual steering angles of the axles are insufficient due to a resistance to turning of the truck and the vehicle body. Therefore, if the steering angle is set at the radial steering angle, the axles do not turn to such an extent that they point to a center of curvature “C” of the curved track.
To address the above issue, Patent Reference 1 proposes a technique of setting the steering angle to an angle that is larger than the radial steering angle. By setting the steering angle at the larger angle, it is possible to compensate for the insufficiency in the steering angle due to resistance in various parts such as resistance between the vehicle body and the truck, resistance within the steering device, and resistance within an axle box support device.
When the set steering angle is larger than the radial steering angle as disclosed in the technique of Patent Reference 1, at the center of the curved track, a lateral pressure from an outer rail on a front axle of a front truck of the railway vehicle reduces. In the following explanation, in a railway vehicle equipped with two trucks, one in the front and the other in the rear of the railway vehicle, each having two sets of axles, the axles will be referred to as a first axle, a second axle, a third axle, and a fourth axle in order from front to rear.
However, even in the technique proposed in Patent Reference 1, the fact remains that the front and rear axles are turned symmetrically. Therefore, when the railway vehicle enters a straight portion at an exit of the curved track (hereinafter, “exit straight portion”), as shown in FIG. 15, the railway vehicle enters in an over-steered posture, whereby the lateral pressure from the inner rail on the first axle increases. In FIG. 15, 2a represents the front truck, 2b represents a rear truck, 3a represents the first axle, 3b represents the second axle, 3c represents the third axle, 3d represents the fourth axle, 4a represents the inner rail, and 4b represents the outer rail.